Conventionally, there have been known ultrasound diagnostic devices in which an oscillating probe having a large number of arrayed transducers is provided; ultrasound is transmitted to and received from a tested subject such as a living body; and on the basis of a signal having been obtained from received ultrasound, ultrasound image data is produced to display an ultrasound image based thereon on an image display device.
In such ultrasound diagnostic devices, ultrasound having been reflected from the same reflection object in electronic scanning has different arrival time with respect to each transducer due to transducer arrangement. In the conventional ultrasound diagnostic device, to correct this arrival time lag, ultrasound having been received with respect to each transducer is converted as an electronic signal, followed by beam forming processing to produce a signal for image formation. This beam forming processing is to adjust the time lag of each signal in which based on the geometric focal distances of transducers, delay correction is carried out for a delay amount having been set with respect to each transducer (each channel).
According to the conventional ultrasound diagnostic device, ideally, the same signal is expected to be obtained from every channel However, the acoustic velocity of ultrasound in a tested subject is not always constant, and actual delay amount may differ from the theoretical value. Therefore, all signals are not always subjected to correct beam forming. Thereby, image data with low S/N is eventually produced.
In view of such problems, a method to determine coherence factor as an indicator showing the quality of a signal having been subjected to beam forming is proposed. This coherence factor is calculated by the ratio of coherent sum to incoherent sum. An increase in this value indicates an excellent quality signal having been subjected to almost ideal beam forming. In contract, a decrease in the value indicates a poor quality signal in which in beam forming, a substantial error has been generated. Then, a thus-obtained coherence factor is applied to a signal having been subjected to beam forming and thereby weighing based on signal quality can be realized to produce image data in which artifacts are suppressed and S/N is enhanced (for example, Pai-Chi Li and Meng-Lin Li, Adaptive Imaging Using the Generalized Coherence Factor, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 50 (2003), No. 2, pp. 128-141).
However, when weighing using a coherent factor is merely carried out to produce image data, for example, in a portion where an artifact such as sidelobe or speckle has emerged, weighed amount is locally minimized and thereby a so-called black defect is generated and then an unnatural ultrasound image may be shown. Thereby, a misdiagnosis by the reader may result and in some occasions, an inappropriate ultrasound image is eventually obtained.
In contrast, in the conventional ultrasound diagnostic device, there is proposed one in which feedback is performed so that the above coherence factor shows a more ideal value to adjust the delay amount per channel during transmission and reception (for example, U.S. Patent Application Publication No. 2005/0228279 specification).
However, in the technique described in U.S. Patent Application Publication No. 2005/0228279 specification, using a coherence factor, excellent quality image data of enhanced S/N can be produced but a circuit structure to adjust the delay amount per channel is required, resulting in an extremely complicated structure.
An object of the present invention is to provide an ultrasound diagnostic device in which, with a simple configuration, image data in which black defects are reduced and S/N is improved can be produced.